Latex paints have gained market share in architectural coatings, e.g., because of environmental concerns with solvent-based paints. Latex paints have desirable performance in many applications, particularly for architectural coatings. However, there are some properties of latex paints that cannot match those of solvent-based paints. One major concern is controlling the rheology of latex paints as rheology relates to, e.g., their flow and leveling properties. The excellent flow and leveling properties of solvent-based paints can provide a smooth finish that is substantially free of brush marks; this is particularly desirable for high gloss paints. Latex paints have been unable to achieve flow and leveling performance comparable to solvent-based paints because of the limitations of various types of thickeners used in latex paints.
There are several types of thickeners, also known as rheology modifiers or rheology additives, for coatings such as latex paints. One type is the non-associative thickeners made of high molecular weight water-soluble polymers. Non-associative thickeners interact with the aqueous phase of latex paints. Exemplary non-associative thickeners include derivatives comprising cellulose, such as those disclosed in U.S. Pat. No. 3,769,247.
Another type of thickener is known as an associative thickener, such as a hydrophobically modified ethylene oxide urethane rheology modifier (“HEUR”). It is well known that hydrophobic segments of associative thickeners associate with hydrophobic groups of a polymer latex, pigments, and other ingredients of latex paints. Thus, they form a physically cross-linked, shear stress reversible, three-dimensional network.
An additional type of thickener incorporates features of the two types described above by interacting with the aqueous phase while hydrophobically associating with hydrophobic ingredients of latex paints. This type of thickener typically contains hydrophilic polymer chains that interact strongly with water. Attached to these polymer chains are monomers bearing hydrophobic groups that can associate with hydrophobic paint ingredients. Hydrophobically modified alkali swellable/soluble emulsions (“HASE”), each emulsion containing a hydrophobically modified alkali swellable/soluble copolymer (“HASC”), and hydrophobically modified cellulosic thickeners are examples of this type. The number average molecular weight of these dual-acting thickeners is often lower that the number average molecular weight of non-associative thickeners but higher than the number average molecular weight of associative thickeners, such as HEURs.
The good flow and leveling, good shear stability, and minimal impact on water sensitivity are recognized advantages of paints containing HEURs and some latex paints formulated with HEURs can have flow and leveling performance approaching that of solvent paints. However, HEURs are, notably and disadvantageously, very sensitive to the paint's composition. For example, changes in the latex, pigment, surfactant, solvent and/or other paint ingredients can have a profound impact on the viscosity of a paint containing a HEUR.
Colorants are a major factor limiting HEUR's usage in tint-base paints. For example, HEURs can only be used in white paints or in light-color base paints. Deep-color base paints with HEURs are known to have disadvantageous viscosity instability, especially low-shear viscosity instability, after they are tinted, i.e., admixed with a universal color concentrate or colorant composition. Colorant compositions contain organic or inorganic pigments, solvents (which can include water), and a large amount of a so-called “conventional” surfactant or surfactants, i.e., not the copolymer surfactants of the present invention. The conventional surfactant(s) of the colorant compositions interact with HEURs, resulting in an undesirable break-down of physically cross-linked, three-dimensional networks in paints, and thereby significantly decrease the low-shear viscosity of such paints. In the high-shear viscosity range, where the viscosity largely depends on the amount of thickener(s) present in the tint-base, colorants can also reduce the high-shear viscosity through their dilution effect on the tint-base.
U.S. Pat. No. 6,337,366 disclosed a mixture of monophobe and multiphobe modified polyethylene oxide added to colorants to preserve the Stormer viscosity stability of tint-bases.
Using hydrophobically modified alkali soluble polymers as thickeners developed in the 1970s. U.S. Pat. No. 4,138,381 discloses a process of making such polymers through solution polymerization. These are copolymers containing carboxylic acid groups that impart water-solubility to the polymers through neutralization. The hydrophobic segments are alkyl or alkyl phenyl (containing from 1 to 20 carbon atoms) ethylene oxide (meth)acrylate. U.S. Pat. No. 4,138,381 disclosed a process of making polymer thickeners through emulsion polymerization in an aqueous phase; such polymer thickeners are now known as HASE. The latex polymer thus prepared is ready to dissolve in water by adjusting the pH to at least 5.5. Similar systems are disclosed in U.S. Pat. Nos. 4,421,902; 4,423,199; 4,432,881; 4,529,773; 4,569,965; 4,600,761; 4,616,074; and 4,892,916. A hydrophobic monomer known as either tristyrylphenylpoly(ethyleneoxy) methacrylate or tristyrylpoly(ethyleneoxy) methacrylate is disclosed in U.S. Pat. No. 5,770,760.